MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) placed in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The above-mentioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a magnetic resonance signal.
In an MRI apparatus, a three dimensional coordinate system composed of three coordinate axes which are orthogonal to each other around the apparatus is defined, in order to define a position and an angle while using the apparatus. This coordinate system is specific for each apparatus and is referred to as an apparatus coordinate system. However, because a patient as an object is imaged in various postures, a coordinate system on the basis of the posture and a direction of the patient is set independently of the apparatus coordinate system.
Then, in an MRI apparatus, a variable coordinate system is determined for each imaging on the basis of (a) the posture of the patient on a table such as a face-up position, a face-down position, a right lateral decubitus position, and a left lateral decubitus position and (b) the direction of inserting the patient into inside of the gantry such as from the head first and from the foot first. This coordinate system is determined on the basis of the posture of the patient at the time of imaging and is referred to as a patient coordinate system.
As an example in this specification, the X axis, the Y axis and the Z axis of the patient coordinate system are defined as follows. The right-to-left (horizontal) direction of the patient is defined as the X axis direction. In addition, the front-back direction of the patient is defined as the Y axis direction, assuming that the abdominal side is the front and the dorsal side is the back. In addition, topside-to-downside direction of the patient is defined as the Z axis direction, assuming that the head side is the topside and the foot side is the downside along the direction of a straight-line approximation of the backbone.
In addition, an X-Y plane of the patient coordinate system is called an axial plane, an X-Z plane of the patient coordinate system is called a coronal plane, and a Y-Z plane of the patient coordinate system is called a sagittal plane. In many cases, a patient is loaded on a table in such a manner that the horizontal moving direction of the table, which is the Z axis direction of the apparatus coordinate system, accords with the Z axis direction of the patient coordinate system.
In an MRI apparatus, orientation information displayed together with an image is determined on the basis of a cross-sectional direction of the patient coordinate system. When an axial cross-sectional image is displayed, for example, characters of “A”, “P”, “R”, and “L” are displayed. Specifically, “A” is displayed near the middle of the upper side of the outer edge of the image, “P” is displayed near the middle of the lower side, “R” is displayed near the middle of the right side and “L” is displayed near the middle of the left side, for example. Here, “A” indicates anterior (the front side of the patient), “P” indicates posterior (the back side of the patient), “R” indicates right (the right side of the patient), and “L” indicates left (the left side of the patient).
In addition, when a sagittal cross-sectional image is displayed, each character is displayed like “H” near the middle of the upper side of the outer edge of the image, “F” near the middle of the lower side, “P” near the middle of the right side, and “A” near the middle of the left side, for example. Here, “H” indicates the head (the head side of the patient) and “F” indicates the foot (the foot side of the patient). In addition, when a coronal cross-sectional image is displayed, each character is displayed like “H” near the middle of the upper side of the outer edge of the image, “F” near the middle of the lower side, “L” (the left side of the patient himself/herself) near the middle of the right side, and “R” (the right side of the patient himself/herself) near the middle of the left side, for example.
Incidentally, as technology for precisely displaying orientation information of a patient associated with an image, Japanese Patent Application Laid-open (KOKAI) Publication No. 2011-183069 is known. In addition, the information indicating the orientation of the generated image such as the above “A”, “P”, etc. is stored as a part (accompanying information) of image data of the generated image, and is used when these image data are transferred from the MRI apparatus to another device.
Formerly, cases where cross-sections in parallel with each other are imaged are prevalent. However, in recent years, cases of imaging from various directions in accordance with an imaging part and a lesion area have been increasing. For example, there is a case of changing the cross-sectional direction of each image in a fan shape around a certain point as the center, or the like. In such multi-angle imaging, because the cross-sectional direction is different from one image to another, which of the axial cross-section, the coronal cross-section, and the sagittal cross-section is the closest to the imaging cross-section is determined for each image based on which posture the patient on the table of the MRI apparatus was inserted in. Then, the orientation information associated with an image is displayed in accordance with the closest cross-section out of the axial cross-section, the coronal cross-section and sagittal cross-section.
In other words, in conventional multi-angle imaging, though the orientation information of A, P, R, and L is additionally displayed as an axial cross-section up to an image of a certain cross-section, the display aspect from the next image suddenly switches into that of a sagittal cross-section or coronal cross-section in some cases, for example. In this case, because the display aspect of the entire image changes, it is difficult for a user having focused on the same region of interest between the respective images such as a cross-section of a spine etc. to continuously observe or visually recognize change of the patient between plural images.
Therefore, novel technology to visually recognize change of a patient between plural images of MRI more easily than conventional technology in the case of changing the cross-sectional direction between these images has been desired.
Similarly, novel technology to visually recognize change of a region of interest between plural images of MRI more easily than conventional technology in the case of changing arrangement of the region of interest between these images has been desired.